Geo: Gitlab::Geo should not serialize ActiveRecord objects

Relating to https://gitlab.com/gitlab-org/gitlab-ce/issues/38490 and https://gitlab.com/gitlab-org/gitlab-ce/issues/55034, Gitlab::Geo in lib/gitlab/geo.rb caches a number of ActiveRecord objects, which breaks when Rails 4 and Rails 5 are deployed in a HA environment.

A quick fix for 11.6 may be to add the RAILS_VERSION to the cache key because I don't think there are significant cache invalidation issues involved, but we should avoid putting binary ActiveRecord objects in there in the first place.